The invention herein relates to processes for recovering platinum group metals in concentrated form from their ores. More particularly, it relates to a process for recovering a platinum group metal concentrate from an ore containing, in addition to the platinum group metals, nickel, copper and iron.
Platinum group metals (hereinafter sometimes referred to as "PGM") may be associated in ores and their concentrates with iron, copper and nickel sulfides. It has been known in the past to separate and refine these ores by conventional pyrometallurgical techniques, in which the various other materials (notably iron) are removed at intermediate steps in the process, leaving a PGM residue as the end product. A typical example is illustrated in U.S. Pat. No. 1,841,207. However, such processes generate large amounts of sulfurous gas emissions, notably SO.sub.2 in combination with minor amounts of other sulfur oxides, and they have thus become environmentally unacceptable unless complex and expensive pollution control devices are incorporated into the processes to control the sulfur emissions.
In U.S. Pat. No. 3,293,027 a two-stage hydrometallurgical process is shown where the principal aim is to recover nickel and cobalt from a ground metal matte containing 60 to 75% nickel, lesser amounts of cobalt and copper, and small quantities of platinum group metals, iron, sulfur, and especially arsenic. A preliminary hydrometallurgical treatment is used to separate the precious metals in a residue which also contains the bulk of the arsenic and iron, as well as some nickel and other non-ferrous metals. Further treatment (such as cyanidation) is thus required to separate the precious metals from the iron and arsenic. U.S. Pat. No. 1,896,807 illustrates collection of the platinum group metals in a high copper content matte and then further refinement to separate the precious metals from the copper. U.S. Pat. No. 1,863,807 illustrates separation of a matte containing copper, lead, iron and precious metals from an ore also containing cobalt, nickel, arsenic and antimony. U.S. Pat. No. 2,425,760 illustrates separation by controlled cooling of a molten matte of copper and nickel in the form of sulfides from a metal alloy containing the precious metals. U.S. Pat. No. 2,777,764 illustrates a hydrometallurgical process for dissolving refractory ores and recovering precious metals therefrom by cyanidation. U.S. Pat. No. 2,829,967 discloses a process for separating platinum group metals from copper-iron-nickel sulfide ores by roasting the non-precious metals to oxides, chlorides or sulfates, leaching the salts to separate the iron and PGM, then chloride treating the solution to separate the iron and finally precipitating the PGM with a non-precious metal, such as copper or iron. U.S. Pat. No. 1,634,497 discloses a process for two-stage roasting and leaching of copper-PGM ores to separate the metals. U.S. Pat. No. 3,767,760 discloses a process for acid leaching and aqua regia separation of PGM from copper ores. Other patents of interest illustrating separation processes having some relation to the present process for obtaining platinum group metals include U.S. Pat. Nos. 1,542,935; 1,983,274; 3,332,771; 3,576,620; 3,785,944; 3,793,430; 3,798,304; 3,816,105; and 3,879,272.
Also of pertinence is "The Initial Development of Processes for the Direct Leaching of Iron-Nickel-Copper Mattes Containing Platinum-Group Metals and for the Production of Ferronickel," by J. P. Martin et al. Report No. 1720, National Institute for Metallurgy, Johannesburg, South Africa (Mar. 7, 1975). This report describes a process in which PGM are partially separated from an ore containing copper, iron and nickel by a series of steps involving smelting, granulation and atmospheric acid leaching with dilute sulfuric acid. In this process hydrogen sulfide is formed and the PGM are not fully separated but rather are retained in a major copper sulfide component.